A conventional pneumatic power tool comprises a spindle with a socket into which a drill bit may be inserted. The spindle is conventionally driven by two separate gears, which are normally arranged as sleeves coaxially around the spindle. A first gear, the drive gear, is arranged in axial splines in order to drive the rotation of the spindle. The second gear, the feed gear, is threaded onto the spindle such that the mutual rotation between the feed gear and the spindle creates an axial movement there between that feeds the spindle in an axial direction. The feed gear may be geared so as to either advance or retract spindle.
Conventionally, the spindle is advanced in that the threaded feed gear rotates at a slightly higher rotational speed than the spindle. When the drilling operation is concluded the feed gear is positioned into an angularly blocked position. Thereby, as the feed gear is blocked from rotation, the continued rotation of the spindle with respect to the feed gear will rapidly retract the spindle back to its initial position.
A problem inherent in such conventional pneumatic power tools is that the spindle and/or the gear that feeds the spindle might get jammed due to the relatively high rotational speed of the spindle and the gears. This is troublesome not only because it implies that the jammed part needs to be loosened, but also because the parts of the tool will be exposed to heavy stress and wear.
Conventionally, this problem is solved by means of a pneumatic signal which is actuated during the last stages of the retraction phase in order to stop the motor by closing the main valve. The actuation of the pneumatic signal is triggered when the spindle has been retracted to a certain point, close to its fully retracted position. The optimal location of this point is governed by the intended rotational speed of the spindle. Typically, the location should be sufficiently close to the fully retracted position in order to assure that the inherent rotational energy of the spindle will be enough to bring it all the way back to the fully retracted position. On the other hand the location should be sufficiently far away from the fully retracted position in order to assure that the rotational speed is not too high such that the spindle will be jammed.
Hence, in the conventional technique, a compromise needs to be made in order for the tool to be adapted to different rotational speeds. This compromise of course means that for rotational speeds in the higher range the spindle will be jammed, and/or that for rotational speeds in the lower range the spindle may not be fully retracted.
An additional problem with the conventional arrangement is that the physical provision of the sensor for actuation of the pneumatic signal builds on the length of the head of the tool. Generally, it is always desired to keep the length of the head as short as possible in order to improve the workability of the tool in confined spaces.
Hence, there is a need for an arrangement that allows the spindle to be safely retracted regardless of the rotational speed of the spindle.